Energy conversion, utilization and access underlie many of the great challenges of our time, including those associated with sustainability, environmental quality, security, and poverty. New applications of emerging technologies are required to respond to these challenges. Biotechnology, one of the most powerful of the emerging technologies, can give rise to important new energy conversion processes. Plant biomass and derivatives thereof are a resource for the biological conversion of energy to forms useful to humanity.
Among forms of plant biomass, both grain-based biomass and lignocellulosic biomass (collectively “biomass”) are well-suited for energy applications. Each feedstock has advantages and disadvantages. For example, because of its large-scale availability, low cost, and environmentally benign production lignocellulosic biomass has gained attention as a viable feed source for biofuel production. In particular, many energy production and utilization cycles based on cellulosic biomass have near-zero greenhouse gas emissions on a life-cycle basis.
However, grain-based feed stocks are more readily converted to fuels by existing microorganisms, although grain-based feed stock is more expensive than lignocellulosic feed stock and conversion to fuel competes with alternative uses for the grain.
Biomass processing schemes involving enzymatic or microbial hydrolysis commonly involve four biologically mediated transformations: (1) the production of saccharolytic enzymes (cellulases and hemicellulases); (2) the hydrolysis of carbohydrate components present in pretreated biomass to sugars; (3) the fermentation of hexose sugars (e.g., glucose, mannose, and galactose); and (4) the fermentation of pentose sugars (e.g., xylose and arabinose). These four transformations can occur in a single step in a process configuration called consolidated bioprocessing (“CBP”), which is distinguished from other less highly integrated configurations in that it does not involve a dedicated process step for cellulase and/or hemicellulase production.
CBP offers the potential for lower cost and higher efficiency than processes featuring dedicated cellulase production. The benefits result in part from avoided capital costs, substrate and other raw materials, and utilities associated with cellulase production. In addition, several factors support the realization of higher rates of hydrolysis, and hence reduced reactor volume and capital investment using CBP, including enzyme-microbe synergy and the use of thermophilic organisms and/or complexed cellulase systems. Moreover, cellulose-adherent cellulolytic microorganisms are likely to compete successfully for products of cellulose hydrolysis with non-adhered microbes, e.g., contaminants. Successful competition of desirable microbes increases the stability of industrial processes based on microbial cellulose utilization. Progress in developing CBP-enabling microorganisms is being made through two strategies: engineering naturally occurring cellulolytic microorganisms to improve product-related properties, such as yield and titer; and engineering non-cellulolytic organisms that exhibit high product yields and titers to express a heterologous cellulase and hemicellulase system enabling cellulose and hemicellulose utilization.
One way to meet the demand for ethanol production is to convert sugars found in biomass, i.e., materials such as agricultural wastes, corn hulls, corncobs, cellulosic materials, and the like to produce ethanol. Efficient biomass conversion in large-scale industrial applications requires a microorganism that is able to tolerate high concentrations of sugar and ethanol, and which is able to ferment more than one sugar simultaneously.
Bakers' yeast (Saccharomyces cerevisiae) is the preferred microorganism for the production of ethanol (Hahn-Hagerdat, B., et al., Adv. Biochem. Eng. Hiotechnol.73, 53- 84 (2001)). Attributes in favor of this microbe are (i) high productivity at close to theoretical yields (0.51 g ethanol produced/g glucose used), (ii) high osmo- and ethanol tolerance, (in) natural robustness in industrial processes, also (iv) being generally regarded as safe (GRAS) due to its long association with wine and bread making, and beer brewing. Furthermore, S. cerevisiae exhibits tolerance to inhibitors commonly found in hydrolysates resulting from biomass pretreatment. Exemplary metabolic pathways for the production of ethanol are depicted in FIG. 1. However, S. cerevisiae does not naturally break, down components of cellulose, nor does it efficiently use pentose sugars,
Glycerol is a metabolic end-product of native yeast ethanolic fermentation (FIG. 1). During anaerobic growth on carbohydrates, production of ethanol and carbon dioxide is redox neutral, while the reactions that create cell biomass and associated carbon dioxide are more oxidized relative to carbohydrates. The production of glycerol, which is more reduced relative to carbohydrates, functions as an electron sink to off-set cell biomass formation, so that overall redox neutrality is conserved. This is essential from a theoretical consideration of conservation of mass, and in practice strains unable to produce glycerol are unable (or only very poorly able) to grow under anaerobic conditions.
There is a strong commercial incentive not to produce glycerol as a byproduct because it represents lost ethanol yield. In industrial corn ethanol fermentations, this yield loss can be up to 6% of theoretical, for a market of ˜14 billion gallons/yr. At selling price of $2.50/gal, this is a total market value of $2 B/yr.
Strategies from the literature to address this problem include decreasing glycerol formation by engineering ammonia fixation to function with NADH instead of NADPH via up-regulation of GLN1, encoding glutamine synthetase, or GLT1, encoding glutamate synthase with deletion of GDH1, encoding the NADPH-dependent glutamate dehydrogenase. (Nissen, T. L., et al., Metabolic Engineering 2: 69-77 (2000)). Another strategy engineering cells to produce excess NADPH during glycolysis via expression of a NADPH linked glyceraldehyde-3-phosphate dehydrogenase. (Bro, C., et al., Metabolic Engineering 8: 102-111 (2006)). Another strategy contained a deletion of GDH1, and over-expression of glutamate synthase (GLT1) and glutamine synthase (GLN1), which also resulted in reduced glycerol formation. However, growth rates and biomass formation were well below the control strain and improvements on the initial performance have not been demonstrated. Additionally, industrially relevant yields, titers and fermentation rates were never demonstrated. (U.S. Pat. No. 7,018,829). Another strategy describes deletion of only GDH1 without overexpression of GDH2 or GLT1/GLN1. However, the strategy was dependent on the use of an industrial polyploid yeast strain capable of tolerating high ethanol concentrations. It is noted in the patent that GDH1 was the only deletion, and that there are no heterologous DNA sequences in the genome. Additionally, the maximum reduction in glycerol production seen was 12.04%, and the technology was not demonstrated on an industrially relevant substrate (U.S. Pat. No. 7,935,514). Most glycerol reduction strategies either only partially reduce the requirement for glycerol formation, or create a by-product other than ethanol. The present invention overcomes the shortcomings of these other strategies.
Corn mash contains free amino nitrogen. However the amount is too low to enable yeast biomass formation sufficient to meet the needs of the process. Nitrogen is added to industrial corn ethanol fermentations to promote yeast growth, most commonly in the form of urea and ammonia. Excess nitrogen can improve the fermentation kinetics of conventional yeasts; however ethanol yields are often lower due to excess biomass and glycerol formation. Typically, urea is added to industrial corn fermentations in concentrations that range from 500 ppm to 1000 ppm.
Yeast take up and assimilate ammonium as its preferred nitrogen source, followed by amino acids, and finally urea (FIGS. 2-4) (extensively reviewed by Lungdhal et al., (Genetics 190: 885-929 (2012)). The mechanism of nitrogen catabolite repression (NCR) control is established by transcription factors which control the expression of ammonium, amino acid and urea transporters. These transcription factors also control expression of proteins responsible for degradation and assimilation of nitrogen containing molecules. It has been shown that de-repression of non-preferred nitrogen source assimilation pathways can improve fermentation kinetics (Salmon, J. M., and Barre, P., Appl. Environ. Microbiol. 64:3831-3837 (1998)); however, effects on ethanol productivity were not measured.
S. cerevisiae contains three known ammonium transporters, MEP1, MEP2 and MEP3. MEP1 and MEP2 are high affinity transporters while MEP3 is a low affinity transporter. S. cerevisiae breaks down urea through the enzymatic action of a urea-amido lyase (EC 6.3.4.6). This activity is encoded by the enzyme DUR1/2 in S. cerevisiae (FIGS. 2-4). Overexpression of DUR1/2 in wine yeasts has been shown to enhance urea degradation rates during fermentation of grape must (Coulon, J., et al., Am. J. Enol. Vitic. 57:2 (2006)). There are two known urea transporters in S. cerevisiae, DUR3 and DUR4 (FIGS. 2-4). It has been shown that overexpression of DUR3 resulted in improved urea degradation rates during wine fermentation (Dahabieh, M. S., el al., Am. J. Enol. Vitic. 60:4 (2009)). U.S. Patent Publ. No. 2011/0129566 describes the expression of DUR1/2 and DUR3 in wine yeasts.
Industrial corn mash substrates contain as much as 3% protein (w/v); however, much of the amino acid content contained in these proteins is unavailable to S. cerevisiae. Expression of one or more proteases would release amino acids that could serve as a nitrogen source for yeast. Additionally, the use of amino acids as a nitrogen source for S. cerevisiae in corn ethanol fermentations would improve yield through a reduction in the surplus NADH generated from in vivo amino acid synthesis during anaerobic growth.
Guo et al. engineered S. cerevisiae to express a heterologous protease for the purpose of improving ethanol yield (Guo, Z-p, et al., Enzyme and Microbial Technology 48: 148-154 (2011)). However, the work was conducted in a wild type yeast background that had not been previously engineered for reduced glycerol formation, and the activity of the expressed endoprotease primarily breaks protein into short polypeptides which are not transported by S. cerevisiae. 
One aspect of the present invention relates to improved fermentation performance through co-expression of an exoprotease to release single amino acids. Additionally, corn kernel protein is primarily a class of storage proteins known as zeins. Zeins have been shown to be recalcitrant to hydrolysis by many proteases and it is possible that expression of zein specific proteases would result in improved proteolysis. Thus, another aspect of the present invention relates to expressing zein-specific proteases to improve corn protein hydrolysis and amino acid utilization by the yeast.
Amino acids are transported by a large family of amino acid permeases. One aspect of this invention relates to deregulation or over-expression of a specific or general amino acid permease to complement protease expression or metabolic engineering by improving the uptake rate of free amino acids released during proteolysis. For example, expression of the general amino acid permease GAP1 is negatively regulated by AUA1. One aspect of this invention relates to the deletion of AUA1 or over expression of GAP1 that could result in improved amino acid uptake rates.
PCT/US2012/032443, which is incorporated herein by reference, teaches a method of eliminating glycerol formation through the production of formate. The formate production pathway can also be combined with strains engineered for reduced activity of the native glycerol production pathway. These combinations can be designed such that strains are built with different degrees of glycerol reduction as shown in FIG. 5. Several embodiments of the current invention relate to a combination of those or related genetic modifications described in PCT/US2012/032443, with additional genetic modifications that are designed to alter nitrogen transport and assimilation.
One aspect of this invention relates to strains of S. cerevisiae with reduced glycerol production that get a kinetic benefit from higher nitrogen concentration without sacrificing ethanol yield. A second aspect of the invention relates to metabolic modifications resulting in altered transport and/or intracellular metabolism of nitrogen sources present in corn mash.